The present invention relates generally to apparatus for rapidly and efficiently producing large quantities of dense pellets from injected, liquified compressed gases such as carbon dioxide (CO.sub.2).
Carbon dioxide is a gas under ambient conditions, has a boiling point of -109.degree. F. and can exist simultaneously in solid, liquid and gaseous phases at its triple point (i.e., at a temperature of -69.83.degree. F. and a pressure of 75.13 psia). When compressed under pressure, CO.sub.2 gas liquifies and such liquified CO.sub.2 may be transformed into solid crystalline particles by injecting the material through an orifice or nozzle into an extrusion chamber at a lower pressure, such as ambient pressure, whereupon the triple point for CO.sub.2 is reached, and the liquified CO.sub.2 is converted into crystalline particles or "snow." During this phase transformation process, significant volumes of gaseous CO.sub.2 boil off, and are vented to the atmosphere, or may be captured, reliquified, and used once again to form snow.
Many applications for the resulting CO.sub.2 snow particles are known in the trade. For example, the snow may be used to directly contact or surround food to refrigerate or quick freeze it, and sublimated CO.sub.2 vapor is recovered. See, e.g., U.S. Pat. Nos. 4,137,723 issued to Tyree, Jr., and 5,170,631 issued to Lang et al. Food may also be frozen by using a liquid CO.sub.2 cryogen as a heat exchange medium. See, e.g., U.S. Pat. Nos. 4,165,618 issued to Tyree, Jr., and 4,356,707 issued to Tyree, Jr. et al.
Carbon dioxide snow can likewise be used to cool equipment parts that cannot be lubricated, and therefore are subject to frictional heat buildup. Moreover, the snow can be sprayed onto chemical spills or leaking fluids to freeze them for pickup before they seep into the ground. Furthermore, burning liquids can be cooled below their ignition temperature, and blanketed above by CO.sub.2 vapor to extinguish fires.
Another market for CO.sub.2 pellets is as blasting media for removing paint or other debris from buildings, road surfaces, airplanes, etc. In this regard, sand particles or other types of grit traditionally, have been blasted at stone or concrete surfaces to remove such unwanted debris. For example, U.K. Application No. 2,077,157 published in the name of Parfloor Ltd. discloses an apparatus and process for treating roadway surfaces in which cryogenic liquid CO.sub.2 is contacted with the road surface to cool it, and freeze any tire rubber, oil, etc., thereupon, after which it is bombarded with cooled shot, grit, or other suitable particles to remove the frozen residue. While the liquid CO.sub.2 sublimates and evaporates upon contact with the road surface, the bombarded shot or grit must be collected for removal. Therefore, this process requires messy clean up of the solid particle medium.
Moreover, use of shot or grit with comparatively delicate surfaces like paint covered aircraft fuselages would be unsuitable, since it would severely damage the underlying fuselage surface in addition to removing the unwanted paint. Therefore, liquid chemicals have traditionally been employed in such applications to soften the paint, after which high-pressure water is used to remove it, so the aircraft may be properly inspected for cracks in the skin. However, large quantities of paint sludge are generated in this process, which must be disposed of in compliance with increasingly stringent and expensive environmental guidelines.
Mercer Engineering Research Center of Warner Robins, Ge. has developed a process for iceblasting the fuselage skin with dry ice particles in which the CO.sub.2 pellets sublimate upon contact with the fuselage, thereby, leaving only the paint chips to be swept up for disposal. While U.S. Pat. Nos. 5,009,240 issued to Levi, and No. 4,655,847 issued to Ichinoseki et al. teach methods for cleaning semiconductor wafers or other substrates with bombarded ice (H.sub.2 O) pellets, or a combination of ice and dry ice particles, it has been found that dry ice pellets are more suitable due to their ready sublimation properties.
It is known in the trade to flash liquid CO.sub.2 maintained at 300 psi and 0.degree. F. into a snow formation horn maintained at atmospheric pressure. Due to the sudden drop in pressure, a portion of the liquid CO.sub.2 crystallizes from its liquid phase to a solid "snow" phase. Attendant with the snow formation is a significant amount of CO.sub.2 gas that boils off as the triple point for CO.sub.2 is reached. U.S. Pat. No. 4,390,356 issued to Preiss et al. discloses such a transformation in a separator cone of a cyclone-type CO.sub.2 snow generator in which CO.sub.2 liquid is injected against flexible polyethylene or polypropylene cone walls to induce vibrations that reduce snow build-up upon the walls. U.S. Pat. No. 4,377,402 issued to Crowe et al. teaches refrigerated temperature control of the injected liquid CO.sub.2 feed in order to increase the production of CO.sub.2 snow in the snow horn. Similarly, U.S. Pat. No. 4,111,671 issued to Williamson illustrates a curved profile for the snow horn to enhance separation of the snow from the horn.
The lumps of CO.sub.2 snow may then be fed to a grinding mill to produce particles of a desired size, as taught by U.S. Pat. No. 4,707,951 issued to Gibot et al. However, it is very difficult to control the shape of resulting ground particles. Moreover, such a process in which CO.sub.2 snow is accumulated in lumps which are subsequently ground are unlikely to have the requisite density for use in sandblasting applications.
An alternative approach is to form small CO.sub.2 snow flakes in a small expansion chamber, which are then conveyed to a larger expansion chamber in which numerous snowflakes agglomerate into large CO.sub.2 snow particles, as disclosed by U.S. Pat. No. 5,125,979 issued to Swain et al. Once again, however, it is difficult to produce by means of agglomeration the kind of particles having uniform size, shape, and density characteristics needed for sand-blasting applications.
A better method for preparing CO.sub.2 snow pellets, therefore, is by extrusion of compacted CO.sub.2 snow. One of the earliest CO.sub.2 pellet extruders was commercialized by Liquid Carbonic Corporation of Chicago, Ill. in 1969. As shown schematically in FIG. 1 herein, extruder 10 comprised an extrusion chamber 12 having a 6-inch bore (inside diameter) with an opening 14 along a portion of the top surface thereof. Mounted over opening 14 was a chimney 16. Mounted to the one open end of extrusion chamber 12 by means of reinforced bolts 18 was die plate 20.
Positioned inside extrusion chamber 12 was piston head 22, which was reciprocated by means of piston shaft 24 and hydraulic cylinder 26. Liquid CO.sub.2 under high pressure was delivered to chimney 16 by means of inlet nozzle 28 and conduit 30, whereupon the transformation of the liquid CO.sub.2 at the triple point previously discussed occurred to form CO.sub.2 snow. Resulting CO.sub.2 gas was vented through outlet stack 32. Upon retraction of piston head 22 within extrusion chamber 12 by hydraulic cylinder 26, CO.sub.2 snow in chimney 16 fell by means of gravity into extrusion chamber 12. Excess CO.sub.2 gas not immediately expelled through vent 32 assisted in pushing the CO.sub.2 snow into extrusion chamber 12. As soon as hydraulic cylinder 26 reciprocated piston head 22 forward in extrusion chamber 12 once again, the CO.sub.2 snow contained therein was compacted and extruded through holes 34 machined into die plate 20 in a single motion to produce dry ice pellets 36.
This Liquid Carbonics extruder suffered from several deficiencies. First, the CO.sub.2 snow needed to be compacted by 75% of its volume in order to increase the density enough for proper extrusion. This compaction force required enormous power in the form of hydraulic cylinder 26 having a 10-inch bore, 30-inch stroke, and 300 gal hydraulic reservoir with 90 gallon and 45 gallon pumps. The extrusion housing 11 enclosing extrusion chamber 12, along with die plate 20 and bolts 18 needed to be reinforced to withstand the tremendous pressures applied by hydraulic cylinder 26.
Second, because of the extremely long 30-inch stroke of hydraulic cylinder 26 and the resulting length of piston shaft 24, web plate 37 was positioned between hydraulic cylinder 26 and extruder housing 11 to accommodate the requisite distance between the two components. Because of the thrust discharged by hydraulic cylinder 26, tie rods 38 and 39 were used to connect hydraulic cylinder 26 and extruder housing 11, respectively, to web plate 37 in order to avoid misalignment of the parts that might interfere with the calibrated movement of piston head 22 in extrusion chamber 12. Tie rods 38 and 39 had to be secured thereto by bolts rotated by 1350 ft-lbs of torque.
Third, extruder 10 was very inefficient in terms of producing dry ice pellets 36, because most of the long 30-inch piston head movement was devoted to compacting the CO.sub.2 snow, not extruding dry ice pellets. Production of dry ice pellets 36 was also limited by the time required to deliver CO.sub.2 snow formed in the 10-12 foot tall chimney 16 to extrusion chamber 12 by means of gravity (with a small assist from the residual CO.sub.2 gas in chimney 16). It was only because of the large scale of the extruder and resulting amount of CO.sub.2 snow that could be formed in chimney 16 that these compaction and extrusion inefficiencies were overcome sufficiently to produce approximately 1000 lbs/hr of dry ice pellets. The resulting extruder 10 weighed 10,000 lbs and lacked any degree of portability and, therefore, was generally installed only as stationary equipment for operating purposes.
FIG. 2 herein shows a slightly more efficient design for an extruder 40 in the form of a single path, linear, sequential pellet extruder. As exemplified by U.S. Pat. No. 5,109,636 issued to Lloyd et al., and a device currently sold by Tomco, the extruder or cylinder housing 42 contains an extrusion chamber 44 with a 3 1/4-inch diameter bore and similarly sized piston head 46 positioned therein. Mounted to the front end is die plate 48 having a plurality of holes 50 machined therethrough. Die plate 48 is typically 11/2 inches thick. The holes 50 are chosen for the desired dry ice pellet 52 diameter, and typically vary between 0.004 inches and 1.0 inches in diameter.
Mounted to the back end of extruder housing 42 by means of web plate 54 is hydraulic cylinder 56 that operates piston shaft 58 connected to piston head 46 with a 13-inch stroke. Once again, because of the thrust forces applied by hydraulic cylinder 56, 5/8-inch diameter tie rods 59 and 60 under 100 ft-lbs of torque are used to connect hydraulic cylinder 56 and extruder housing 42, respectively, to web plate 54 in order to ensure proper alignment of piston head 46 with extrusion chamber 44.
Hydraulic cylinder 56 is controlled electronically and opens directional flow valves to enable hydraulic fluid to be pumped from and be returned to a reservoir, as is known in the trade. The fluid actuates the hydraulic cylinder by charging and exhausting oil from each side of the cylinder piston.
In a conventional dry ice pellet extrusion cycle for the Tomco in-line, linear or single path, sequential extruder 40, liquid CO.sub.2 under high pressure is injected from tank 62 directly into extrusion chamber 44 once control valve 63 is opened. The liquid CO.sub.2 transforms phases to form snow in the extrusion chamber, while venting CO.sub.2 gas through exhaust port 64. Next, hydraulic cylinder 56 moves piston head 46 forward in extrusion chamber 44 to compact a mass of CO.sub.2 snow formed therein against die plate 48 to increase the density thereof. By pushing the compacted CO.sub.2 snow through holes 50 in die plate 48, dry ice pellets 52 are extruded.
While such an in-line, single path or linear, sequential extruder 40 is more efficient than the Liquid Carbonics machine 20, because the CO.sub.2 snow is formed directly in the extrusion chamber 44, instead of in a separate chimney 16 from which the snow must be transported by gravity, it still suffers from several problems that limit the extent to which it can produce dry ice pellets 52. First, because the liquid CO.sub.2 injection, CO.sub.2 snow formation, CO.sub.2 gas venting, CO.sub.2 snow compaction, and dry ice pellet extrusion functions occur sequentially in the machine, the extruder 40 is limited to approximately one cycle per minute (i.e., roughly 35 seconds to inject the liquid CO.sub.2, form snow, and vent the CO.sub.2 gas, and 25 seconds to move the hydraulic cylinder sufficiently to compact the CO.sub.2 snow, extrude dry ice pellets, and retract the piston head from the extrusion chamber). These time requirements are due in part to delays in crystal particle formation due to inadequate gas venting, delays due to compaction inefficiencies, and delays due to slow transfer of hydraulic fluid during the charging and exhaustion stages of the hydraulic cylinder 56. At approximately 7-8 lbs of dry ice pellets 52 produced per cycle, single path extruder 40 can produce a maximum of 120 lbs dry ice pellets each hour.
Second, carbon dioxide gas produced during the CO.sub.2 snow formation is discharged through vent holes limited to the top of the surface compaction/extrusion cylinder. However, since approximately 1.5 lbs of CO.sub.2 gas is formed for each 1 lb of CO.sub.2 snow, the small planar vent area reduces CO.sub.2 snow production substantially. Moreover, a metal screen having 5-micron sized holes needed to be placed over the vent holes on the extrusion cylinder to contain the CO.sub.2 snow in the compaction/extrusion cylinder during the compaction stage. These vent holes frequently clogged with CO.sub.2 snow, thereby further limiting venting of CO.sub.2 gas, and the consequent amount of CO.sub.2 snow that can be formed in the cylinder.
Third, the Tomco extruder 40 uses a die plate 70 shown in more detail in FIG. 3, having tapered holes 72 conically drilled therethrough. While this configuration further compacts the CO.sub.2 snow as it is pushed through the holes 72 during the extrusion stage to further increase the density of the resulting dry ice pellets, it is very expensive to drill conical holes through a 11/2-inch thick metal die plate. This expense is compounded by the need for a minimum of 50% net open area across a 31/4-inch diameter die plate 70 to produce the resulting 120 lbs dry ice per hour.
Fourth, the single path extruder 40 still requires tie rods 59 and 60 to accomplish precision alignment of hydraulic cylinder 56, web plate 54, and extrusion housing 42, using long wrenches to apply the required 100 ft-lbs of torque to the tie rods, because of the large size and weight of the components. This assembly process is cumbersome, and poses safety risks if a tie rod should break under such extreme pressures.
While the 120 lbs/hr production capacity of the Tomco extruder 40 is far less than the 1000 lbs/hr production rate of the Liquid Carbonics extruder 10, it is achieved with a far smaller machine that is more portable. Efforts have been made by Tomco, however, to scale up the components in its linear path, sequential extruder 40 to achieve higher dry ice pellet production rates. For example, an extrusion chamber having a 6-inch diameter bore and piston head has been combined with a massive hydraulic cylinder with a 24-inch stroke. Using 1-inch diameter tie rods under 650 ft-lbs of torque to align the hydraulic cylinder, web plate, and extrusion housing, as well as a 11/2-inch thick die plate, 480-500 lbs/hr of food processing grade dry ice pellets have been obtained. In order to achieve the same enhanced production rate of the denser, sandblasting grade pellets, however, an 8-inch diameter extrusion chamber and piston head were needed in combination with a 24-inch-stroke hydraulic cylinder, 2-inch-thick die plate, and 11/4-inch tie rods at 1350 ft-lbs of torque.
It will be readily appreciated that such efforts to increase production rates through a mere increase in component size produces many disadvantages. Because of the extremely large size of the resulting overall extruder system, the Tomco 6-inch-diameter extruder weighs 4500 lbs and is powered with a stationary electrically wired motor, thereby making it impossible to use it in the field where portability is key. The 8-inch-diameter Tomco extruder for sandblasting grade dry ice pellets is even heavier. Moreover, the amount of torque that must be applied to the tie rods is extraordinary. The 650 ft-lbs needed for Tomco's 6-inch diameter extruder is applied using a 66-inch-long wrench. At 1350 ft-lbs for Tomco's 8-inch-diameter extruder, the wrench and the consequent effort by and danger to a maintenance employee is even greater. Furthermore, the 11/2 to 2-inch-thick die plates greatly increase machining costs. Finally, separate extruders are required to produce sandblasting and food process grade pellets. Thus, 480-500 lbs/hr of dry ice production by Tomco's scaled-up extruder comes at a price.
Other examples of single path, linear, sequential pellet extruders are disclosed in U.S. Pat. Nos. 3,576,112; 3,618,330; 3,708,993; and 4,780,119. Each of these products suffer from the same problems noted above relative to the Tomco extruder.
Efforts have also been made in the trade to extrude dry ice pellets without a piston head operated by a hydraulic cylinder. For instance, in U.S. Pat. No. 4,389,820 issued to Fong et al., CO.sub.2 snow formed in a snow chamber is forced by orbiting rollers through an annular, donut-shaped die, having radially extending holes through its circumference. Pins extending partially across the path of the die holes help to direct the extruded CO.sub.2 snow, and break them off into dry ice pellets of predetermined length. U.S. Pat. No. 4,977,910 issued to Miyahara et al. similarly discusses the use of a rotating barrel positioned inside an outer barrel with die holes along its perimeter to force CO.sub.2 snow through the holes to produce dry ice pellets. U.S. Pat. No. 4,033,736 issued to Cann uses rotary means to force CO.sub.2 snow into an extrusion chamber against opposed forces applied by springs to extrude dry ice pellets, while stabilizing them as additional CO.sub.2 gas is emitted. Finally, U.S. Pat. No. 3,670,516 issued to Duron et al. discloses a rotary extruder in which CO.sub.2 snow is compacted between cooperating teeth along the perimeter of a circular die ring and rotating gear pinions, and then forced through draw holes located in the die ring to extrude dry ice pellets.
However, it is believed that such rotary driven extruders are much more complicated to build, operate, and maintain due to the large number of intricate parts.
As has been noted, in many applications for dry ice pellets, it would be beneficial to use a feed exceeding 500 lbs of dry ice pellets per hour. Conventional equipment manufacturers attempted such a goal by combining two of its 6-inch diameter extruders out of phase to produce one overall machine producing approximately 1000 lbs/hr of food processing-grade dry ice pellets. Operation of the respective hydraulic cylinders were coordinated so that while CO.sub.2 snow was being formed in one compaction/extrusion chamber, CO.sub.2 snow was being compacted and extruded in the other chamber. While such an arrangement provided, in theory, nearly two cycles per minute, and therefore 1000 lbs/hr of dry ice pellets, the machine weighed 6500 lbs, and required complex measures to mechanically integrate and control the functions of each 6-inch extruder barrel in a cooperative relationship.
In summary, conventional dry ice machines are sized to accommodate the rate of snow formation, gaseous CO.sub.2 removal, snow compaction and extrusion. The rate of snow formation is dependent on the rate; therefore, the amount of gaseous CO.sub.2 removed from the transformation reaction. In this regard, prior art machines suffer from a deficiency in the rate of removal of gaseous CO.sub.2 from the chamber, which contains and separates the snow from the gas. For example, in such prior devices, approximately one and one-half (11/2) pounds of gaseous CO.sub.2 must be separated and removed for one (1) pound of snow production.
One manufacturer of conventional machines, drills two hundred twenty-six (226) holes that are 0.201 inches in diameter in a one-half (1/2) inch thick steel tube that serves as the collection, compaction and extrusion chamber for a dry ice pelletizer. A fine mesh screen covers the drilled holes. This configuration results in a net open surface area of nine and one-half percent (9.5%) but is incapable of providing sufficiently rapid production rates for optimal commercial acceptance.
Sintered metal filters have been proposed in the patent literature (e.g., see U.S. Pat. No. 3,576,112 to Frost and U.S. Pat. No. 3,618,330 to Hardt et al) for use in filtering and separating gaseous CO.sub.2 from snow in dry ice pelletizing machines. However, such sintered metal filters have not been incorporated in commercially successful machines because of blinding or blockage deficiencies and breakage due to embrittlement at low transformation temperatures.
It is believed that the problems encountered in using sintered metals as filter media for venting dry ice pelletizers is due to the single grade metal particles employed and the uniform particle size and the distribution of particles utilized in preparing such sintered materials. In this regard, it is to be noted that conventional sintered metals are formed in a manner such that uniformly straight and consistent air passageways are distributed throughout the surface of the material. No accommodation is made for configuring the passageways in tortuous or irregularly shaped fashion.
In view of the construction of the prior art machines and, in particular, the composition and configuration of the filter media employed therein for venting the extrusion chamber, the efficiency of manufacture and the rate of production of dry ice particles utilizing these machines has been adversely impacted.